Overview & Objectives

RAD Science Overview

The Radiation Assessment Detector (RAD) investigation is an investigation to detect and analyze the most biologically hazardous energetic particle radiation on the Martian surface as part of the Mars Science Laboratory (MSL) mission. There are two primary types of energetic particle radiation incident at the top of the Mars atmosphere, Galactic Cosmic Rays (GCRs) and Solar Energetic Particles (SEPs). Both GCRs and SEPs interact with the atmosphere and penetrate into the soil where they produce secondary particles – including neutrons and γ-rays – that contribute to the radiation environment on the surface. This radiation environment, and the associated health risks to astronauts, will have a major impact on the planning of any future human missions to Mars.

RAD Science Objectives

The primary science objectives of the RAD investigation are:

To measure energetic particle spectra at the surface of Mars

Measurement of the composition and intensity of GCRs, SEPs, and their interaction products for the first time on the surface of Mars is the primary objective of the RAD investigation. The charged and neutral particle spectra on the surface of Mars are functions of both the incident flux and the effects of transport through the atmosphere and top layers of soil. Providing "ground truth" observations also enables the ability to test radiation transport models.

To measure dose and determine dose equivalent rates for human explorers on the surface of Mars

RAD will make the first direct measurements of the radiation environment on the surface of Mars, which is essential for planning future human missions. Future astronauts conducting Martian surface operations will experience a continuous exposure to GCR radiation, and potentially large but short-duration exposures from SEPs.

To use these measurements to enable validation of Mars atmospheric transmission models and radiation transport codes

Space radiation transport models play a central role in radiation risk assessment for astronauts. This is due in part to the complexity of the underlying physics, particularly the nuclear interactions, where not all of the ingredients of a model can be taken from measurements. For example, many nuclear cross sections have not been measured and must be estimated, and this lack of data may introduce significant biases and/or uncertainties (Townsend et al., 1990). In addition, atmospheric conditions on Mars, including seasonal changes in composition, due for example, to condensation of CO2 and variable regolith properties, will modulate the surface radiation environment further. RAD will provide "ground truth" observations enabling scientists to test these radiation transport models.

To provide input to the determination of the radiation hazard and potential mutagenic influences to life at or just below the Martian surface

The radiation effects on potential indigenous Martian life forms (past and present) are unknown, but most current studies assume that life elsewhere will be based on polymeric organic molecules (Pace, 2001), and will in an overall sense, share with terrestrial life the vulnerability to energetic radiation. Thus the risks to extant organisms are assumed to be analogous to the risks to future human explorers. Energetic particles ionize molecules along their tracks, creating OH and other damaging free radicals. More specifically, energetic particles can modify or even break DNA strands within cells, with the surviving cells becoming cancerous.

RAD will quantify the flux of biologically hazardous radiation at the surface of Mars today, and measure how these fluxes vary on diurnal, seasonal, solar cycle and episodic (flare, storm) timescales. These measurements will allow calculations (with validated transport models) of the depth in rock or soil to which this flux provides a lethal dose for known terrestrial organisms. Through such measurements, we can learn, for example, how deep life would have to be today to be protected from the Mars radiation environment.

To provide input to the determination of the chemical and isotopic effects of energetic particles on the Martian surface and atmosphere

On geological time scales, an enormous fluence of high-energy charged particles (both primary and secondary) has interacted with, and possibly altered, the Martian regolith. If estimates of the present dose equivalent rate can be extrapolated back for as little as 107 years, the surface and shallow sub-surface have received on the order of a few million Sieverts (Sv) of radiation. This raises the possibility that ionizing radiation may have contributed significantly to the unique chemistry of the Martian surface. RAD measurements of the SEP and GCR flux at the surface of Mars will enable an interpretation of high-energy particle weathering of the Martian surface.